The goal of this study is to elucidate the molecular mechanisms by which lac repressor interacts with lac operator. The lactose operon of E. coli is chosen as a system in which the general principles of regulation of transcription can most easily be studied. Genetic methods will be used to isolate and characterize specific repressor mutants (a) which specifically overcome Oc mutations, and (b) which have an increased affinity for wild-type operator. One class of repressor mutations which overcomes Oc mutations and of which some showed a largely increased affinity for wild-type and Oc operators in an in vitro test will be further investigated. DNA sequencing will be used to analyze several Oc mutations and a "super operator" mutation. A secondary binding site for the repressor will be mutated to increase its affinity for the repressor and make it function as an effective operator. My long-range concern is to identify the amino acid residues of the lac repressor and the base pairs of the lac operator which contribute to the repressor-operator interaction.